A printing unit has a plurality of rotatable printing cylinders. An offset printing unit, for example, has a plate cylinder and a blanket cylinder. The plate cylinder and the blanket cylinder are supported at their opposite ends for rotation in the frame of the printing unit. The plate cylinder carries a printing plate having a surface on which an inked image is defined. The blanket cylinder carries a printing blanket. When the cylinders rotate in the printing unit, the plate on the plate cylinder transfers the inked image to the blanket on the blanket cylinder at a nip between the plate cylinder and the blanket cylinder. The blanket on the blanket cylinder subsequently transfers the inked image to the material being printed, such as a web of paper.
A printing unit typically includes a skew mechanism to skew the plate cylinder relative to the blanket cylinder. When the plate cylinder is skewed relative to the blanket cylinder, the angular position of the plate on the plate cylinder is changed relative to the blanket on the blanket cylinder. The angular position of the inked image which is transferred from the plate to the blanket, and subsequently to the web, is likewise changed. The alignment of the image with the web is thus adjusted by adjusting the skew mechanism.
An offset printing unit also includes a throw-off mechanism. When the printing plate and/or the printing blanket are to be serviced or replaced, the cylinders are moved relative to each other into thrown-off positions. When the cylinders are in their thrown-off positions, they are spaced from each other to permit access to the printing plate, the printing blanket and the web.